


The Heroine Appears

by Quantum_Knight



Category: Undertale (Video Game), 僕のヒーローアカデミア | Boku no Hero Academia | My Hero Academia
Genre: Alternate Universe - Canon, Crossover, Gen
Language: English
Status: In-Progress
Published: 2019-06-26
Updated: 2019-06-26
Packaged: 2020-05-20 11:19:34
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,492
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/19375663
Author URL: https://archiveofourown.org/users/Quantum_Knight/pseuds/Quantum_Knight
Summary: Undyne, Heroine of the Underground and the Spear of Justice, is reborn into the Japan of My Hero Academia. On her journey to become a hero, she'll show the world just how determined monsters can be.





	The Heroine Appears

Her cerulean hands glided across the ivory piano keys like smoothed stones tossed across a still pond. Practiced fingertips pressed into the heart of each individual note. They sang their songs in delighted reprise, eager to meld their music together in aquatic symphony as the girl strung them along piece by piece.The instrument before her calloused appendages displayed signs of wear of its own. Commonly used keys bore nearly invisible marks where the girl had used them countless times before. The dull mahogany of the piano's outer casing possessed numerous etches and scratches where her claws had brushed by in frustration while playing a particularly stubborn sonnet. The bronze pedals below her feet were smoothed into a dull, dark gray.

The girl's eyes were trained on a brown paperback set squarely atop the piano's casing. The book was opened to a page near the end, accented by musical notes arranged much like stars across a night sky. Her brow was furled in deep concentration as she scanned the material whilst ordering her fingers to comply with what was instructed to exact specifications.

Ding. The noise interrupted her train of thought like a fishing rod shoved into an intricate machination of gears. Her hands halted on the keys they lied on. A long moment passed after that final note, with only the ticking of a nearby clock providing any noise in the small room.

Then, quite like instant noodles prepared in a microwave set on high for twenty minutes, everything boiled over and exploded rather spectacularly.

"NGAAAAAAAH!"

The fish monster's clawed fist collided with the instrument like a meteorite, the white keys squeaking out a pained garble in response. The book leapt from its stand and soared majestically into the distance.

"Dammit! Seriously, on the last note? I was so close…" She jumped from her wooden seat, pointing a challenging finger at the inanimate object. Her lips curled sour as she snarled,

"You think you're better than me, huh? Well, just you wait! One more spar, and I'll get the best of you for sure." The girl's breathing became ragged as she stared her opponent down. Thoroughly intimidated, the piano couldn't respond.

Satisfied, the girl slammed her clenched fist into her chest and roared in victory. The tall glass of water resting uncomfortably on the outer edge of the piano vibrated like a fervent cicada in the height of summer.

"Fufufu! If that's all you got, let's go for round-"

"-Undyne!" A painfully adolescent voice squawked from across the room, interrupting Undyne's challenge. Her eyes rolled as she spun around towards the awkward individual now perched in her room's doorway. His matted, curly green hair frayed wildly about his head, drawing immediate attention. His black middle school uniform, usually pressed and free of blemishes, was wrinkled and curt, with gray strings dangling from the collar and cuffs. His shoelaces were scattered about like spaghetti flung across a dinner table. His green eyes confirmed the stress his appearance suggested as he took a deep breath in.

Undyne's brow furled in annoyance as she began, "Izuku! What did I say about just barging into my-"

"W-wait! I'm sorry, but it's important!"

Undyne scoffed as she crossed her arms, "What could possibly be so important that you'd interrupt my battle with-"

"T-t-the clock!"

Unydne's frustration fizzled away, replaced by stark confusion. She turned her head to observe the old digital clock resting on a nearby desk.

"Yeah, what about it?"

No sooner had she asked the question than she discovered its unfortunate answer.

7:48.

The gears fell into place. A thick chill sank through her fishy body as she felt her heart descend into the deep dark depths below. Her pointed teeth clamped into a shaky smile as she turned again to face the boy trembling in the doorway.

"Aha. I see."

A few moments passed as the pair stood in silence. Finally, Izuku gathered the courage to speak.

"So-"

"-NGAAAAAAH! Why didn't you tell me sooner?"

Undyne sprang into action, dashing across the room and into her closet in a bur of blue and red. Various articles of clothing flew out of the aperture at breakneck pace, landing on every conceivable surface within Undyne's bedroom. The somewhat sweaty teenager wasn't spared from the onslaught, quickly buried in T-shirts of vibrant colors and designs. Undyne muttered hushed expletives as Izuku attempted to remove the mountain assailing his small frame.

"My alarm didn't ring, so I only got up a few minutes ago, myself. I must've turned it off, somehow." Izuku's voice was muffled under a particularly baggy shirt.

Another flash of cerulean and scarlet whirled by the boy like a hurricane. The mound of clothing was blasted away as Undyne's afterimage slid down the hallway, exiting the room. Blinking, Izuku turned to pursue the monster as fast as his relatively short legs could carry him.

"Where's the leftover ramen? 'Know what? Forget it; looks like we're gonna have to skip breakfast." Undyne said in annoyance as she filled a large glass with white milk. She never stopped floating about the house as she pounded the glass's contents back in one massive gulp, little droplets of liquid splattering about the wooden floor. Izuku tried to maintain his eccentric adopted sibling's pace as best he could, sighing as he resigned to an empty belly.

Another clock read 7:52 as Undyne retrieved her black backpack from a desk and made a final push for the door leading outside. Izuku followed suit, sprinting past the concerned and perplexed woman seated on a coach.

"Bye, mom!" Izuku squeaked out, his breath nearly failing as his lungs begged him to slow down. Inko Midoryia's brow furled in consternation, but she nevertheless smiled back, calling out to the duo,

"Have a good day at school, but hurry. You're gonna be late!"

"We will, bye." Undyne hurriedly shouted back as she opened the wooden door. Clutching Izuku's wrist, she threw him from the hallway and into the outside world. Thus, the marathon began.

The bright sun above shone unobstructed on the pair as they charged through the empty streets. The air was crisp, but pleasant, with a warm breeze blowing Undyne's scarlet hair messily about. Dew drops twinkled like stars in the grassy patches on either side of the road. Golden flowers swayed gently in the wind. Only the sound of footsteps on asphalt and ragged breathing from the boy being towed along accompanied their journey towards the squat building in the distance. A few minutes of running later, and the two found themselves at the border separating the neighborhood from the middle school jutting into blue sky above. Taking a moment to catch her breath, Undyne retrieved a silver pocket watch from her uniform's top. A grin exploded across her face as she proclaimed to the world,

"Victory is mine! 'Made it with seven minutes to spare. How's them apples?"

Izuku knew his sister expected some kind of reply; however, in his current state, he could do nothing but place his hands on his knees, bend over and gasp for air. His legs screamed in pain, unused to such sustained exercise. After a few seconds, he composed himself enough to exhale in reply,

"Pretty…good…I guess."

Undyne harrumphed, satisfied, her toothy grin growing ever wider. Izuku weakly smiled back, straightening upwards as his eyes scanned the schoolhouse. The rambunctiousness of that particular morning had prevented the boy from processing the laborious task that would be the school day ahead. However, with the immediate threat of tardiness dispelled, the dread that normally occupied Izuku's waking thoughts came rushing back as a stone of anxiety plopped into his gut. An image of a certain hotheaded explosion Quirk user flashed in his mind. He audibly groaned and rolled his eyes, forcing his legs to march towards the building ahead.

Undyne followed suit, but her smile faded as concern for her brother overwrote the joy of past victory.

"Hey, buddy, don't you worry. Not only are we gonna be on time, but we're going to make today the greatest day in the history of ever." She playfully punched Izuku's shoulder, the boy recoiling in surprise. Rubbing the impact area, he sighed in reply.

"Uh-huh. You say that every day."

Undyne raised her clenched fist into the air, another smile exploding across her features as she shouted, "That's because it's true!"

Izuku couldn't stop the grin creeping across his face. The day ahead seemed bleak, filled with the usual insults and pestering of a blond haired maniac, but his sister's presence made the sky above seem a little bluer.

Besides, there were still important things to learn in order to become a hero.

\---

"So, as third year students, it's time to start thinking seriously about your futures and what you want to do with your lives." The middle aged man in front of the class droned on, much to the dismay of the blue fish lady squirming in her desk. The group of twenty-odd black uniformed student's all stared at the teacher with varying levels of interest, some struggling to keep their eyes open, others whispering hushed questions to each other, and one green haired individual in particular studiously taking notes in a tan paperback.

"I could pass out some career aptitude tests, but…why bother." The teacher finished his statement by dramatically tossing the stack of papers in his hands into the air, continuing on,

"I know you all want to go to the Hero track!"

The classroom erupted into a storm of voices raised in anticipation, the students shouting out in glee and freely using their wildly varying powers. The teacher did his best to bring his class back down to ground level. With the end of the school year fast approaching and the prospect of becoming a hero turning into a reality for his students, the teacher the reason for his students' excitement. Nevertheless, unauthorized quirk usage within school grounds was a no-go.

Among the shouts, an uninterested yet condescending voice demanded respect from both the teacher and his fellow students. "Hey, Teach, don't lump me in with this bunch of losers. I'm the real deal, but these guys will be lucky to end up as sidekicks to some busted D-lister. Heh."

Katsuki Bakugo sat in his seat with his legs crossed over the desk like the proud king of a vast empire. A usual cocky grin plastered across his face, he was impossibly happy to see the class erupt in protest to his challenge. Annoyance spread across Undyne's features as she observed her opponent. The blond brat absorbed the teacher's praise like roots slurping up water from the dirt. She couldn't stop the grimace from forming on her face, nor did she prevent a low growl from exiting her throat.

After the teacher remarked on Katsuki's ability, and even the possibility of his acceptance into U.A. High, the premier hero school of Japan, he continued along in his egotistic tirade, "That's exactly why it's the only place worthy of me."

Leaping from his seat and onto his desk with clenched fists, he gloated, "I aced all of the mock tests. I'm the only one at this school who stands a chance of getting in. I'll end up more popular than All-Might himself, and be the richest hero of all time. Evil all across the world will know who I am. And it all starts with U.A. High!"

Undyne rolled her crimson eyes, sighing dramatically. The blond would-be hero cricked his head towards the blue-skinned student, the look in his eyes hurling fiery daggers in her direction. He opened his mouth to speak before being cut off by the teacher. Reading from a list of his students' preferred school of choice, he offhandedly remarked, "Oh yeah, Midoriya, don't you want to get into U.A. too?"

The room fell deathly silent as Izuku shrunk down to the size of a pebble in his seat, shielding himself from Katsuki's awestruck, piercing gaze. Then, as the green haired boy predicted, the classroom erupted into laughs, jeers, and insults from every direction.

"Midoriya? Get into U.A.? As if!" One student remarked to her compatriot, both of them chuckling to themselves like rabid hyenas.

"There's no way you're getting into the hero course without a Quirk!" Another male student with a stone- shapeshifting Quirk remarked.

Clambering to his feet and stammering awkwardly, Izuku defended himself as best he could, saying, "Well, they actually got rid of that rule. I could be the first one."

A roar erupted like the thunderous wrath of an angry god as the blond hair Explosion Quirk user leveled the startled Quirkless student's desk with a flash of orange light. Izuku was hurled onto his backside as smoke and steam wafted from the splintered remains of the blackened furniture. Bakugo couldn't help but chuckle at the fearful student as he began in growl,

"Listen up, Deku."

Bakugo towered over Izuku, his teeth clenched and eyes aflame with indignant fury. "You're even worse than the rest of these rejects, you Quirkless wannabee. You really think they'd let someone like you in when they could have someone like me?"

Time ground to a halt as a flash of blue light screamed by Bakugo's face, just inches from his pointed nose.

Thock.

A five foot long spear of blue light embedded itself into the drywall to Katsuki's left. Eerie silence followed in its wake as every student in the classroom gazed at the materialized weapon hanging freely in the plaster. Almost mechanically, Katsuki rotated his head to the right, eyes darkened.

"That's it, then. Here I come!"

Long before the dumbfounded Katsuki could even hope to properly react, Undyne charged from across the room, a second spear ready in her hands. Flashing a menacing smile, she halted a foot from the blond haired teen.

"En-guarde!"

OST- SPEAR OF JUSTICE

Survival instincts taking over, Bakugo swung his right arm around, attempting to slam his flaming fist into the furious fish lady's right shoulder. Seeing the attack in her peripherals, Undyne moved her spear upwards just in time to deflect the blow. Angry sparks erupted from the collision point in a flash of bright light and black smoke. The weapon splintered in two, the pieces falling to the ground and dematerializing instantly as the sour smell of napalm wafted into the air. Anticipating a counter attack, Katsuki spun backwards on his toes, retreating out of the immediate range of Undyne's spear. With a shout, she rematerialized an additional weapon, impaling the open air where his shoulder would have been. Heart pounding like a battle-drum in his chest, Katsuki called out to the raging monster before him, his voice tinged with fury and surprise,

"What the hell, fish-lips?" You really wanna do this after what happened last time?"

Growling, Undyne bent her knees into a battle stance, extending her spear in challenge and roaring, "What'd I say about calling him Deku, flame-brain? You've brought this one on yourself!"

Izuku, prone on his back and thoroughly fearful for his life, watched helplessly as the two titans stared the other down. He thought he might attempt to console his sister, but reminded himself of the other occasions when she entered this rage state. Stubborn unto a fault, no amount of counsel could sway the determination coursing through her soul. He gulped. She was on the warpath now. Nothing short of divine intervention could stop her. All he could do was watch, jot down some notes, and perhaps discover just how to tone down Undyne's uncontrollable temper.

Katsuki raised his arms to his sides, palms upwards and spewing smoke and fire. Spittle flung from his mouth as he said,

"Fine by me, then. I've been itching for a good fight!"

"Stop it now, both of you!"

The teacher finally gathered the courage to intervene, calling out to the students in a panicked tone. He held his breath for a few tense moments, hoping against hope that the two hotheads would listen, for a miraculous change of pace. Both students turned to face their instructor, suddenly aware of the damage they had caused and the hot water they'd just fallen into. Begrudgingly, Undyne was the first to lay down her weapon, allowing it to dematerialize harmlessly on the floor. Katsuki followed suit a few seconds later, muttering to himself something about the teacher's hairstyle. The rest of the frightened students, who had been busy barricading themselves within a makeshift desk fortress over the past few minutes, exhaled a collective sigh of relief. Perhaps they'd actually survive.

Undyne crossed her arms, looking down at the floor as she grinned to herself. "Fufufu. Guess you're off the hook this time, hedgehog hair."

Bakugo spit towards the wall and harrumphed, defiant. "I wouldn't count your blessings yet, mackerel breath." He replied, looking away.

Sweat beading his brow, the teacher pointed a finger towards the pair, saying quietly, "You two, with me. Principal's office.

\---

Undyne muttered softly as she meandered down the busy street. Staring at the rough asphalt, she offhandedly observed the numerous potholes and chips of concrete dotting the road. The frustration swirling in her soul drowned out the pleasant feeling of a breeze blowing gently by. Katsuki Bakugo. The name crept into her mind like an unwanted guest. The hotheaded megalomaniac had taken great pleasure in tormenting her little brother for as long as she could remember. As humorous as poking a tiny bit of fun at her Quirkless brother could be, in moderation, Bakugo had taken it too far too many times. Sometimes, when she caught the two interacting alone, she thought she could overhear words as severe as death threats slip out of Bakugo's mouth.

That's what she had tried to explain to the principal, cooped up in his tight, old office with the insufferable brat seated next to her. The white-haired geezer would have none of it, however, remarking on how "good of a student" Bakugo was, and how she was "overreacting" to the blatant mockery of her sibling. He must've been blinded by the prestige he and his school would gain by having a student be accepted into U.A., she reasoned. Undyne grimaced at the ground. Why did everyone have to beat down on Izuku so much? It just didn't make any sense. His Quirklessness didn't make him any less valuable to society. So what if he didn't have any flashy powers?

He just wanted to be a hero.

The anger coursing through her body welled up in her arms. With a shout, she slammed her fist against a nearby stop sign, causing it to groan in protest. "Damn Bakugo, what does he know? Izuku and I, we'll both become heroes. We'll fight side by side, for justice!" She audibly exclaimed to the onlookers running by.

Something pierced through the haze of rage swirling in her head, anchoring her back in reality. Something out of place.

"Why are they running?"

The thought flashed through her mind like a bolt of lightning. Raised voices from down the street became audible as a horde of people coalesced on a street corner, all of their eyes clearly transfixed on a particular thing obstructed from Undyne's view by tall buildings.

Curiosity mingled with dread sank into her soul. This was the path she and Izuku used to get home. She had to stay at the school longer with Bakugo and the principal, leaving her brother to head back an hour or so sooner. She broke out into a sprint, catching up to the crowd within a few moments. She opened her mouth to ask what was going on, only to have that question answered by her own eyes instantly.

Crimson fire scorched the street and jet black smoke hid the blue sky. In the center of the chaos, a dark green mass with bloodshot eyes and gigantic square teeth swirled and gargled, defending itself from costumed heroes operating in tandem to pacify the villain. They appeared to be unable to move in close, prevented by the creature holding some small blonde thing in its maw as hostage.

Her eyes shot wide open as she recognized the struggling individual. Katsuki Bakugo. He didn't have long, she realized, not while he was being suffocated by that monstrosity's vile liquid. Panicked, she spun around, trying to find any more pro heroes. The only ones on the scene appeared unable to do any more help without injuring the boy. A flash of green darted in front of the crowd. The people shouted in protest, urging the boy not to do whatever he was planning. What normal civilian could possibly be so foolish as to engage a villain head-on? She squinted at the blur, and felt her heart dropping into her toes. The only one crazy enough to do something like this… of course, it had to be him.

"Izuku, you idiot, stop!"


End file.
